Monte Cook's Arcana Evolved
I had long been curious about the many role playing game supplement books that appear to be about Dungeons & Dragons but are not published by TSR or Wizards of the Coast. I finally picked this one up at a used book store because it was written by Monte Cook, who was one of the lead designers of the third edition of D&D. It turns out that back in 2000 Wizards of the Coast issued an Open Game License which allowed other publishers to publish materials specifically designed to supplement Dungeons & Dragons, under certain conditions.
Cook's book is halfway between an alternative players handbook and a campaign setting. Like a players handbook it includes new races, classes, spells, and such. It also sets up a setting called the Realm of the Diamond Throne with a rather detailed backstory. The Realm has some nods to the work of Stephen R. Donaldson, particularly with its giants which do not resemble traditional D&D giants. This borrowing from Fantasy authors is not surprising as old D&D drew heavily on Tolkien and to a leaser extent on Vance, Moorcock, Leiber, and Lovecraft.
I am sure the book would be very useful for GM's looking to supplement their campaigns with outside elements. Personally I only read these books for their entertainment value and I found this one quite interesting, especially the elaborate backstory of the giants and the dragons.